Many ventilatory anesthesia apparatuses are known that can be used to perform anesthesia on a patient who is to undergo a surgical intervention or similar, by administering to the patient, by inhalation, a conventional anesthetic gaseous mixture composed of N2O, halogenated agents, for example sevoflurane, isoflurane, desflurane, etc. In this connection, reference may be made to documents EP-A-983 771 and EPA-A-1 120 126.
Xenon is an anesthetic gas that has been known since the start of the 1950s and that is being used more and more in the medical field, especially as it is particularly suitable for anesthesia of weak patients (elderly patients, long operations, cardiac surgery, neurosurgery, etc.), in particular because of the virtual absence of any influence on blood pressure during anesthesia and the virtual absence of side effects or adverse events.
However, anesthesia performed with xenon requires monitoring of the concentrations of xenon in the gaseous flow administered to the patient, that is to say requires that the concentration of xenon in the anesthetic flow can be determined in real time. In this connection, reference may be made, for example, to documents EP-A-1 499 377, EP-A-1 318 797 or EP-A 523 315.
To measure the concentration of xenon in such a gaseous mixture, it is customary to use a mass spectrometer or a chromatograph. These techniques, however, have disadvantages as regards cost and especially as regards the difficulty of implementing them, since their integration in existing anesthesia apparatuses requires considerable efforts in terms of development and adaptation.
An alternative has been proposed in WO-A-2007/068849, which discloses an apparatus for ventilatory anesthesia of a patient by administration of a gas containing gaseous xenon, said apparatus comprising means for determining the xenon concentration so as to determine the content of gaseous xenon in the main gas circuit in the form of an open or closed circuit.
In this apparatus, one or more hot-wire sensors, each having at least one wire made of electrically conductive material, preferably metal, are in direct contact with the gaseous flow containing the xenon, and calculating means cooperate with the hot-wire sensor(s) in such a way as to determine the concentration of xenon in said gaseous flow from a voltage measurement carried out by the voltage-measuring means at the terminals of at least one hot wire or of a resistance placed in series with at least one hot wire, when said at least one hot wire is in contact with the gaseous flow and is traversed by an electric current.
Although this apparatus makes it possible to determine with sufficient precision the concentration of xenon delivered to the patient during gas anesthesia in such a way as to guarantee efficacy of anesthesia and increased safety for the patient, while at the same time being of simple architecture of modest cost, it has been found in practice that, in certain cases, especially in the case of the onset of occlusion that can occur during the course of use through accumulation of humidity in the sampling line or in the case of normal or premature aging of the suction pump, the stability of the measured signal or signals may be adversely affected by the fluctuations in the capacity of the pump for removing the samples.
The reason for this is that, when the sample of gas to be measured is susceptible to variations in flowrate or fluidic oscillations, due for example to the pump for removing the gaseous sample, this is to some extent manifested in disturbances in the measurement of the concentration of xenon.
Although these disturbances lead to a concentration measurement that remains very acceptable, it is desirable to be able to eliminate these disturbances and avoid these fluctuations in the measurement of the content of xenon.
In other words, the problem to be solved is that of improving the apparatus described in WO-A-2007/068849 in such a way as to eliminate all the disturbances in the measurement of the concentration of xenon in the gaseous flow and thereby increase the measurement stability, that is to say provide this device with greater measurement precision, so as to be able to achieve even more effective, reliable and precise monitoring of the concentrations of gaseous xenon in a gaseous anesthesia mixture based on xenon containing, in addition, and in variable quantity, that is to say from 0 to 100% by volume, one or more of the following main compounds: oxygen (O2), nitrogen (N2), nitrous oxide (N2O), carbon dioxide (CO2), halogenated compounds of the isoflurane, enflurane, desflurane, sevoflurane or halothane type, ethanol, and, optionally, traces or small quantities (<1%) of one or more of the following minor compounds: acetone, methane, carbon monoxide (CO), argon, helium, etc.